The advantages with lignin separation from black liquor is already described in WO 2006/031175 and WO02006/038863. These patents disclose the novel process LignoBoost™ that is now sold by Metso, and wherein WO 2006/031175 discloses the basic two stage acidic wash process and WO2006/038863 disclose an improvement of the process where sulphate or sulphate ions are added to the process.
An important aspect of the process is that the required charge of chemicals for the acidification may be high. If this is the case the cost of fresh chemicals is a large part of the operational cost and the commercial viability of the process is lower. These problems could be reduced, if the process is optimized for minimum requirement for charges of fresh chemicals, making the lignin product commercially sound. Acidifiers in form of mill generated waste flows are thus preferable as it may solve a waste disposal problem and lessen environmental impact. As the precipitation of lignin requires acidification of alkaline black liquor flows, much of the total amount of acidifier is used to decrease the pH level down to the point of where lignin starts to precipitate. The first phase reaching this pH level typically reduce the pH level from about pH 13 in the original black liquor down to a pH level about 11.5, and normally do not involve any nucleation of lignin particles. The amount of acidifier needed is nevertheless relatively high for this first phase as the pH follows a logarithmic scale, and any following additional lowering of pH from 11.5 requires far less acidifier for the same order of lowered absolute pH value.
The Lignoboost process produce a lignin product which if used as fuel is classified as a “green” fuel as being based upon recovered fuel. The idea with classification of “green” fuels is based upon the concept not to increase the carbon dioxide footprint, i.e. the emissions, by burning fossil fuels. The most promising acids for this process is carbon dioxide for at least initial precipitation of the lignin, and then using sulfuric acid (H2SO4) for washing and leaching out metals from the lignin. The sulfuric acid could be added as a fresh sulfuric acid from a chemical supplier, or as preferred using so called “spent acid” from a chlorine dioxide generator often used at a pulp mill. The latter usage of this spent acid already at hand in most mill sites further emphasize that the lignin product is considered as a “green” fuel.
Another problem with the process disclosed in WO 2006/031175 is that there may be a disposal problem with the strongly odorous H2S gases that are emitted from the reslurrying tank and bled out from the process, and it is suggested that these hydrogen sulfides could be added to the pulping process in order to increase sulfidity and possibly increase the yield in the pulping process. However, such rerouting of the strongly odorous H2S gases to another part of the pulp mill introduces risks for emissions of these gases during transport and storage. It is far better to use these gases at the location or process producing these gases.
As the chemical constitution of the original black liquor may change during operation, typically due to changes in the pulping process as of changes in wood material used or cooking conditions, the first precipitation process for precipitating lignin particles from the original black liquor may need adaption to the present conditions. As differing requirements apply for the first precipitation phase, where mainly lowering of pH is the objective, and the second precipitation phase, where lignin starts to precipitate it will be difficult to design a system that meets both these requirements.
As the precipitated lignin needs to be separated from the acidified black liquor slurry, which still is kept alkaline at a pH level above 7, it is important that the filterability of the precipitated lignin is high. If the filterability is improved could smaller separation equipment be used, and less investments is needed.